Car battery won't hold a charge: can one estimate how much longer it will last? Or is it best to replace it?

Question

I have a 12V 60Ah EFB battery in a 3-year old start/stop car.

The battery doesn't seem to hold a charge. Is there any way to estimate / understand how much longer it can last, or if it's best to replace it now?

I never left it unused for more than 2 weeks, except now, under quarantine, when I charged it with a charger after about 3 weeks without using it, but I did use it a lot for relatively short trips which may have never fully charged the battery (I didn't measure).

24 hours after a full charge, a multimeter measured 12.35V.

7 days after a full charge, it measured 12.1V. Both times without using the car.

I recharged it again but noticed the same: 12.35V after 1 day and 12.1V after 7 days.

There is no alarm or GPS tracker to justify a huge parasitic drain.

The charger is an Optimate 5 start/stop which can specifically handle EFB batteries.

If I used the car every day for long journeys I wouldn't be worried, but with the world under quarantine I won't, and I am afraid the battery could die just when I need the car.

Any suggestions? Thanks!

Answer 1

Two possibilities:

• The battery indeed needs replacing. As you discovered, old batteries don't die by having reduced cranking amps; they die by having reduced capacity. Thus, the phenomenon you're seeing seems to be very similar to a dying battery.
• There is parasitic drain in the car. Something, some circuit, is left on even though it should be off.

To discover which of these two options is the case, you need to use the amperage functionality of the multimeter. Attach the multimeter in amperage mode between the negative battery post and the negative cable. Close all doors of the car but leave the hood open. See how much drain there is. The drain could be initially quite large, but e.g. 1 hour after not using the car, you should see very little drain. Anything more than 0.1 amperes means the drain is probably too large.

But before using the multimeter in amperage mode, you need to learn how to actually use it. In particular, why it is a bad idea to connect the multimeter to both negative & positive battery posts in ampere mode. If you don't understand this, don't use the amperage mode!

Answer 2

If the battery will not hold a charge, then it's already at the end of its life. It should be replaced right away or as soon as you plan on using the car again.

As it is, chances are good that the car will not start the next time you need to use it.